This invention relates to tufting machines and more particularly to apparatus for driving the pattern cam used for effecting longitudinal displacement of the needle bar of a tufting machine to extend the capabilities thereof.
In the production of tufted fabric it is known to jog or shift the needle bar in its longitudinal direction transversely across the tufting machine relatively to the base material in order to create various pattern effects, to break up the unattractive alignment of the longitudinal rows of tufts and to reduce the affects of streaking which results from variations and colorations of the yarn.
Various devices have been proposed and are in use for controllably applying a step-wise force to the needle bar of the tufting machine in accordance with a pattern. Such needle shifting or stitch placement drives conventionally fall into two categories. The first category is that of the cam driven type whereby a rotating plate cam, driven directly from the tufting machine mainshaft, is drivingly engaged with the needle bar so as to effect the required displacement thereof. The second category is that of the programmable type which may be hydraulically or pneumatically driven, or driven mechanically through some form of programmable indexing device whereby a ram is drivingly engaged with the needle bar so as to effect the required displacement thereof. Examples of such drives are illustrated in U.S. Pat. Nos. 3,964,408 and 3,972,295, which utilize pawl and ratchet devices, U.S. Pat. Nos. 4,010,700 and 4,392,440 which utilize indexing devices and 4,173,192 which uses a hydraulic actuator.
Because of the greater reliability, simplicity and lower cost of a cam drive system, a cam driven needle bar shifter remains the primary drive for supplying the controlled step-wise force to the needle bar in accordance with the information on the periphery of the pattern cam. Examples of such needle shifting devices are disclosed in U.S. Pat. Nos. 3,016,380; 3,934,524 and 4,445,447.
In the conventional cam driven needle bar shifter apparatus, the cam acts on cam followers connected through drive rods and the like to the needle bar and the cam is rotatably driven through proper reduction apparatus from the mainshaft of the tufting machine and rotates at a constant speed in synchronism with the operation of the needles and the hooks or loopers. The cam serves to drive the needle bar in its longitudinal direction during that portion of the machine cycle when the needles are above and out of engagement with the backing material in which the stitches are to be formed so as to avoid interference between the needles and the needle plate. This occurs during only a small portion of the cam circumference so that only this portion of the cam circumference is available for controlling the needle bar movement while the remaining portion of the cam circumference is of a constant radius and non-effective and merely idles the needle bar in place. Thus, the number of needle bar movements for a given cam is limited, and the stitch pattern repeat is similarly restricted due to the relationship between the "dwell" time, i.e., the period within a machine cycle when the needles are engaged with the backing material and no longitudinal shift of the needle bar occurs, and the "displacement" time, when the needles are withdrawn from the backing and the needle bar is jogged.
Typically the needle bar is shifted or jogged laterally across the machine during approximately 120.degree. to 180.degree. of the needle bar reciprocation cycle so that the "displacement" time using a conventional cam driven shifter is approximately 33 percent to 50 percent of the machine cycle and of the circumference of the cam, with the remaining 50 to 67 percent of the cam circumference and machine cycle being an idle surface or the "dwell" time. Thus, for a major part of its rotation the cam is precluded from effecting shifting of the needle bar. If the surface of the cam is divided into sectors equal in number to the number of stitches in the pattern, the angular distance from a point in one sector to a similarly disposed point in an adjacent sector is the angle the cam must rotate for each revolution of the tufting machine mainshaft and for each cycle of the needle bar. Because of this, and because of the small surface available for a follower to ride upon each sector of a practical sized cam, the number of sectors into which the cam may be divided, and hence the number of stitches in a pattern produced by the cam, is limited. Moreover, because of the small time available for needle bar displacement, coarse cam profiles must be utilized for the displacement step, thereby giving rise to problems of inertia in relation to the needle bar and militating against acurate and smooth needle bar movement. Because of the abrupt changes in the shape of the cam surface to produce the required abrupt directional reversals of the movement of the needle bar as the cam rotates, problems arise with regard to ensuring that the cam follower runs smoothly on the cam surface and it is difficult to achieve satisfactory pattern control in the case of high speed tufting machines. Additionally, the need for rapid transition between the "dwell" and "displacement" portions of the total cam profile require that cam followers of relatively small size be utilized, which in itself gives rise to further problems relating to dynamic reponse characteristics.